Method of fabricating CMOS image sensor

ABSTRACT

A method of fabricating a CMOS image sensor is provided, in which a trapezoidal microlens pattern profile is formed to facilitate reflowing the microlens pattern and by which a curvature of the microlens may be enhanced to raise its light-condensing efficiency. The method includes forming a plurality of photodiodes on a semiconductor substrate; forming an insulating interlayer on the semiconductor substrate including the photodiodes; forming a protective layer on the insulating interlayer; forming a plurality of color filters corresponding to the photodiodes; forming a top coating layer on the color filters; forming a microlens pattern on the top coating layer; and forming a plurality of microlenses by reflowing the microlens pattern.

The present application is a divisional of Application No. 11/301,830,filed Dec. 12, 2005, pending. This application also claims the benefitof Korean Patent Application No. 10-2004-0105563, filed on Dec. 14,2004, which is hereby incorporated by reference as if fully set forthherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image sensor, and more particularly,to a method of fabricating a CMOS image sensor. Although the presentinvention is suitable for a wide scope of applications, it isparticularly suitable for enhancing light-condensing efficiency byimproving a curvature of a lens in a CMOS image sensor.

2. Discussion of the Related Art

An image sensor is a semiconductor device that converts an optical imageto an electric signal. Image sensors can be classified intocharge-coupled devices and complementary metal-oxide-semiconductor(CMOS) image sensor devices.

The CMOS image sensor includes a photodiode unit sensing an appliedlight and a CMOS logic circuit unit processing the sensed light into anelectric signal as data. Photosensitivity of the image sensor isenhanced if a quantity of light received by the photodiode is raised. Toenhance the photosensitivity, a fill factor (e.g., the photodiode areadivided by the entire area of image sensor) may be raised, or a path oflight incident on an area other than the photodiode may be diverted tocondense or direct the diverted light to the photodiode.

For example, a microlens may be used in condensing the diverted lightonto the photodiode. By providing a convex microlens formed from amaterial having good light transmittance over a photodiode, a path of anincident light may be refracted towards the photodiode. Hence, morelight can be applied to the photodiode area. In doing so, a lightparallel to an optical axis of the microlens can be refracted by themicrolens to focus at a prescribed position on the optical axis.

Referring to FIG. 1, a CMOS image sensor according to a related artincludes one or more photodiodes 11 formed on a semiconductor substrate(not shown) to generate electric charges according to a quantity ofincident light received on the photodiode(s) 11, an insulatinginterlayer 12 formed over the substrate including the photodiodes 11, aprotective and/or first planarizing layer 13 formed on the insulatinginterlayer 12, a R/G/B color filter layer 14 formed on the layer 13 totransmit light having a specific wavelength therethrough, a secondplanarizing layer 15 formed over the substrate including the colorfilter layer 14, and a microlens 16 formed on the second planarizinglayer 15 having a convex shape with a predetermined curvature andadapted to condense the received light onto an area of the correspondingphotodiode 11 through the color filter layer 14.

An optical shielding layer (not shown) may be provided within theinsulating interlayer 12 to prevent the light from one filter in thecolor filter layer 14 from entering another photodiode area except thecorresponding photodiode 11 (i.e., the photodiode directly below a givenfilter). The photodiode can be replaced by a photo gate to sense thelight.

In this case, a curvature, height (A), and like dimensions of themicrolens 16 are determined by considering various factors including afocus of the condensed light and the like. The microlens 16 is mainlyformed from a polymer-based resin by deposition, patterning, reflowingand the like. Namely, the microlens 16 has a size, position and shaperelated to a corresponding unit pixel and a thickness of thephotosensitive device thereunder, and generally has an optimal sizedetermined according to a height, position, size and the like of theoptical shielding layer, and the radius of curvature.

Meanwhile, the shape of the lens pattern profile depends on an exposurecondition (e.g., focus) of the photoresist. For instance, one or moreprocess conditions for making the microlens can be varied according to athin film situation of a semiconductor substrate, whereby a profile ofthe microlens may be changed. Since the pattern forming condition can berelatively unstable, the light-condensing efficiency can be less thanoptimal. Thus, in the process for fabricating the related art CMOS imagesensor, the microlens 16, provided to enhance a light-condensing power,is an important factor and/or structure affecting characteristics of theimage sensor. A microlens having increased light-condensing efficiencytransmits more light to the photodiode 11 through the correspondingcolor filter layer 14 (which selects a color for detection by photodiodeaccording to the wavelength of light that passes through it). That is,the light incident on the image sensor is condensed by the microlens 16and filtered by the color filter layer 14, and then it enters thephotodiode 11 under the color filter layer 14. In doing so, the opticalshielding layer (not shown) plays a role in preventing the incidentlight from deviating to another light path and entering a differentphotodiode (usually for a different color, and always corresponding to adifferent location in the CMOS image sensor array).

FIG. 2 shows a color filter layer on a protective and/or planarizinglayer in a CMOS image sensor according to a related art. Here, aplurality of rectangular color filter layers 14 are arranged on theprotective and/or planarizing layer 13, uniformly spaced apart from eachother.

However, the convex microlens in the related art CMOS image sensor hasthe following problems.

First of all, in the case of hemispherical microlenses, the lightparallel to the optical axis is refracted by the lens and then arrivesat the photosensitive device, which is located at a position opposingthe lens, to operate the device normally. Yet, light that is notparallel to the optical axis may be refracted by the lens and arrive atanother photosensitive device that is not supposed to receive the light.Hence, the device may operate abnormally.

Secondly, since a quantity of light arriving at the photosensitivedevice varies according to the species and thickness of the layer underthe microlens, the light-condensing efficiency may be reduced anddegrade the quality of the image.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method offabricating a CMOS image sensor that substantially obviates one or moreproblems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a method of fabricatinga CMOS image sensor, in which a trapezoid microlens pattern profile isformed to facilitate a reflow of a microlens and by which a curvature ofthe microlens may be enhanced to raise the light-condensing efficiencyof the microlens.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned from practice of the invention. Theobjectives and other advantages of the invention may be realized andattained by the structure(s) and/or process(es) particularly pointed outin the written description and claims hereof as well as the appendeddrawings.

To achieve these objects and other advantages in accordance with thepurpose of the invention, as embodied and broadly described herein,there is provided a method of fabricating a CMOS image sensor, themethod comprising forming a plurality of photodiodes on a semiconductorsubstrate; forming an insulating interlayer on the semiconductorsubstrate including the photodiodes; forming a protective layer on theinsulating interlayer; forming a plurality of color filterscorresponding to the plurality of photodiodes; forming a top coatinglayer on the plurality of color filters; forming a microlens pattern onthe top coating layer corresponding to the color filters; and forming aplurality of microlenses by reflowing the microlens pattern.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle(s) of theinvention. In the drawings:

FIG. 1 is a cross-sectional diagram of a CMOS image sensor according toa related art, in which a microlens is shown;

FIG. 2 is a layout of a color filter layer on a protective and/orplanarizing layer in a CMOS image sensor according to a related art;

FIG. 3 is a layout of a color filter layer on an insulating interlayerin a CMOS image sensor according to the present invention;

FIG. 4 is a diagram for the comparison of intensity profiles ofultraviolet radiation for exposure in color filter arrays (CFA) betweenthe related art and the present invention; and

FIGS. 5A-5C are cross-sectional diagrams of a method of fabricating aCMOS image sensor according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, like reference designations will be usedthroughout the drawings to refer to the same or similar parts.

FIG. 3 shows a color filter layer on an insulating interlayer in a CMOSimage sensor according to the present invention. Here, a plurality ofcolor filter layers 34 are formed on an insulating interlayer 33,uniformly spaced apart from each other. The spacing in one dimension(e.g., along a first axis in the plane of the page parallel with an edgeof the color filter) may be independent of the spacing in anotherdimension (e.g., along a second axis in the plane of the page,perpendicular to the first axis). In this case, a boundary or edge ofeach of the color filter layers 34 is indented or “stepped.” Also, theinsulating interlayer 33 may further comprise a protective and/orplanarizing layer at an upper surface thereof.

FIG. 4 compares intensity profiles of ultraviolet radiation for exposurein color filter arrays (CFA) between the related art and the presentinvention. Here, a plurality of indented portions are formed on aboundary of a color filter array 34 of the present invention, whereas arelated art color filter array 14 has a rectangular shape.

In the related art method, in performing exposure to form a microlenspattern to correspond to the color filter array 14, a rectangular imageis provided. After completion of development, the microlens pattern hasthe rectangular shape.

In performing exposure to form a microlens pattern corresponding to thecolor filter array 34 having the indented portions on its boundary, theboundary of the color filter array 34 may have a curved or sphericalshape due to its UV intensity being lower than that of a centralportion. Hence, after the plurality of indented portions have beenformed on the boundary of the color filter array, if exposure anddevelopment are carried out to form the microlens pattern, a trapezoidpattern can be formed, as shown in FIG. 5B. Since resolution power atthe boundary of the color filter array 34 having the indented portionsis lowered in performing the exposure, the microlens pattern may have auniform slope. Once reflowing is carried out on the trapezoid microlenspattern, a microlens having an improved or optimal curvature can befabricated to enhance the light-condensing efficiency thereof.

FIGS. 5A-5E show a method of fabricating a CMOS image sensor accordingto the present invention.

Referring to FIG. 5A, a plurality of photosensitive devices such asphotodiodes 31 are formed on (or in) a semiconductor substrate (such asa single crystal silicon wafer, or an epitaxial silicon layer thereon).An insulating interlayer 32 is formed on the semiconductor substrateincluding the photodiodes 31. In this case, the insulating interlayer 32can include a plurality of layers. For instance, a first insulatinginterlayer (not shown) may be formed over the semiconductor substrate,an optical shielding layer formed on the first insulating interlayer toprevent the incident light from entering an area except thecorresponding photodiode 31, and a second insulating interlayer 32formed on the optical shielding layer. Subsequently, a planarizedprotective layer 33 may be formed on the insulating interlayer 32 toprotect a device from moisture and mechanical damage, such as scratches.

A dyeable resist is coated on the protective layer 33. Exposure anddevelopment are carried out on the dyeable resist to form a color filterfor one color (e.g., red, green or blue) in color filter layer 34. Thecolor filter-forming process is repeated for the remaining colors toform color filter layer 34, configured to filter light of differentwavelengths, respectively in the different color filters. In doing so, amask having a pattern enabling indented portions to be formed on aboundary of the color filters in layer 34 is used as a photomask (notshown) during an exposure step in forming the color filter layer 34.

Subsequently, a planarized top coating layer 35 is formed on the colorfilter layer 34, to secure a focus adjustment and a degree ofplanarization for subsequent formation of a lens layer.

Referring to FIG. 5B, a microlens material layer is coated on the topcoating layer 35. The material layer is then patterned by exposure anddevelopment to form a microlens pattern 36 a having a trapezoid shape.In doing so, a resist or an oxide layer such as TEOS can be used as themicrolens material layer.

As mentioned in the foregoing description, after the plurality ofindented portions have been formed on the boundary of the color filterarray, if exposure and development are carried out to form the microlenspattern 36 a, a trapezoid microlens pattern 36 a (e.g., a microlenspattern having a plurality of pre-microlens bodies with trapezoidalcross-sections) may be formed. Since the resolution power on theboundary of the color filter array having the indented portions isconsiderably reduced when exposing the deposited microlens materiallayer to patterning radiation or light, the trapezoidal cross-sectionshapes in microlens pattern 36 a may also have a uniform (or linear)slope.

In the embodiment of the present invention, the trapezoid microlenspattern 36 a is generally formed after completion of the plurality ofindented portions on the boundaries of the color filters in the colorfilter array. Yet, the microlens pattern 36 a can be formed in variousways. For instance, after a microlens material layer (e.g., a resist)has been coated onto layer 35, a shape of the microlens pattern can becontrolled by adjusting the intensity of UV radiation applied during theexposure by passing the UV radiation through a diffraction mask, and thelike. In doing so, the diffraction mask may include an opening, ashielding part and a slit part.

Referring to FIG. 5C, reflowing is carried out on the microlens pattern36 a to form a microlens 36. In doing so, the reflow operation can becarried out by placing the semiconductor substrate with the microlenspattern 36 a thereon on a hot plate or in a furnace. In this case, acurvature of the microlens 36 may be varied or determined according to acontracting-heating method. A light condensing efficiency of themicrolens 36 may depend on its curvature.

Subsequently, ultraviolet radiation is applied to the microlens 36 toharden it. In doing so, applying the ultraviolet radiation to themicrolens 36 may enable the microlens 36 to maintain an optimalcurvature radius.

Accordingly, the present invention provides the following effects oradvantages.

First of all, the microlens is formed from a trapezoidal microlenspattern and reflowing the pattern. Hence, an improved or optimalcurvature may be provided to enhance the light-condensing efficiency ofthe microlens.

Secondly, if the light-condensing efficiency is raised or increased, aquantity of the light passing through the color filter to enter thephotodiode may be increased. Hence, the present invention can enhancethe detection, sensitivity and/or implementation of colors.

Finally, if the detection, sensitivity and/or implementation of colorsis enhanced, the performance of the CMOS image sensor may be improved orenhanced.

It will be apparent to those skilled in the art that variousmodifications can be made in the present invention without departingfrom the spirit or scope of the invention. Thus, it is intended that thepresent invention covers such modifications provided they come withinthe scope of the appended claims and their equivalents.

1. A CMOS image sensor, comprising: a plurality of photodiodes on or ina semiconductor substrate; an insulating layer on the semiconductorsubstrate including the photodiodes; a color filter layer on theinsulating layer, the color filter layer comprising a plurality of colorfilters, each color filter comprising a plurality of indented portionson a plurality of boundaries thereof; and a plurality of microlenses onthe color filter layer, each of the plurality of microlensescorresponding to a unique photodiode.
 2. The CMOS image sensor of claim1, further comprising a coating layer on the color filter layer, whereinthe plurality of microlenses are on the coating layer.
 3. The CMOS imagesensor of claim 1, wherein the microlenses comprise a resist.
 4. TheCMOS image sensor of claim 1, wherein the insulating layer furthercomprises a protective layer on its upper surface.
 5. The CMOS imagesensor of claim 1, wherein the insulating layer further comprises aplanarizing layer on its upper surface.
 6. The CMOS image sensor ofclaim 1, wherein the insulating layer comprises a plurality of layers.7. The CMOS image sensor of claim 1, wherein the insulating layercomprises an optical shielding layer.
 8. The CMOS images sensor of claim1, wherein the insulating layer comprises a first insulating layer, anoptical shielding layer on the first insulating layer, and a secondinsulating layer on the optical shielding layer.
 9. The CMOS imagesensor of claim 1, wherein the plurality of indented portions forms asquare wave pattern.
 10. The CMOS image sensor of claim 1, wherein theplurality of indented portions forms a rectangle wave pattern.
 11. TheCMOS image sensor of claim 1, wherein the spacing of the color filtersin one dimension is independent of the spacing in a perpendiculardimension.
 12. The CMOS image sensor of claim 1, wherein the boundary ofeach color filter has a curved or spherical shape.
 13. The CMOS imagesensor of claim 1, wherein the microlenses have a curvature thatenhances their light-condensing efficiency.
 14. The CMOS image sensor ofclaim 1, wherein the microlenses comprise an oxide.
 15. The CMOS imagesensor of claim 2, wherein the insulating layer further comprises aprotective layer on its upper surface.
 16. The CMOS image sensor ofclaim 2, wherein the insulating layer comprises a plurality of layers.17. The CMOS image sensor of claim 2, wherein the plurality of indentedportions forms a square wave pattern.
 18. The CMOS image sensor of claim2, wherein the plurality of indented portions forms a rectangle wavepattern.
 19. The CMOS image sensor of claim 2, wherein the microlensescomprise a resist.